1. Field of the Invention
The present invention relates to light emitting element modules and chips.
2. Description of the Related Art
There has been known one type of light emitting element module in which a plurality of a rectangular-parallelepiped chips each including plural LEDs (Light Emitting Diodes) (i.e., "LED array") are arranged on a board (this type of light emitting element module is hereinafter referred to as "LED module").
As known in the prior art, an LED array may be constructed of a plurality of light-emitting elements incorporated in a chip so as to make a linear array of the light emitting elements. One well-known fabrication method of an LED array is disclosed in U.S. Pat. No. 5,523,590. The fabrication method disclosed in that reference deposits an insulating film of aluminum oxide (AL.sub.2 O.sub.3) on a wafer comprising an n-type semiconductor substrate; patterns the insulating film by photolithography to form an array of rectangular openings; diffuses a p-type impurity such as zinc through the openings to form an array of p-type diffusion regions in the n-type substrate, thereby creating pn junctions; then deposits an aluminum film on the entire wafer and patterns the aluminum to form a set of electrodes, one electrode making contact with one of the p-type diffusion regions. Each pn junction functions as a light-emitting diode (LED).
When an LED module is applied as a light emitting device, it is used as a light source for an LED print head for use in an electrophotographic printer, for example. On the other hand, when an LED module is applied as a light emitting device having a photosensitive (photodetecting) function as well as a light emitting function, the LED module is also usable as a reading/writing head for reading and writing characters, images, etc.
FIG. 7 is a plan view which schematically shows a general LED module, and shows rectangular chips 70 constituting the LED module and the periphery thereof. Each of the rectangular chips 70 has a first side surface 70a, a second side surface 70b, a third side surface 70c and a fourth side surface 70d.
As shown in FIG. 7, each chip 70 has plural light emitting elements 72 which are arranged so as to be linearly aligned in a longitudinal direction of the chip and displaced (offset) from the longitudinal center line of the chip to one side of the chip. Here, the first side surface 70a is defined as one side surface of the chip 70 at which the light emitting elements 72 are arranged so as to be displaced (offset). The second side surface 70b is defined as another side surface of the chip 70 which intersects to the first side surface 70a. The third side surface 70c is defined as another side surface of the chip 70 which is opposed to the first side surface 70a. The fourth side surface 70d is defined as the residual side surface of the chip 70 which is located between the first side surface 70a and the third side surface 70c, intersects both the first and third side surfaces 70a and 70c, and is opposed to the second side surface 70b.
Accordingly, with respect to the chips 70 of the conventional LED module (light emitting element module), it is understood from FIG. 7 that all the plural light emitting elements 72 of each chip 70 having a substantially rectangular shape in plan view are disposed so as to be displaced (offset) from the center line of the chip 70. These plural light emitting elements 72 are located near to one side surface of the chip 70 (for example, the first side surface 70a), and are linearly aligned with one another.
Furthermore, the chips 70 are arranged at a predetermined chip interval on a board 74 so that an array line L is formed by the plural light emitting elements 72. In this case, the chips 70 are arranged on the board 74 so that the first side surface 70a of each chip 70 is located at one side area with respect to the array line L while the third side surface 70c of each chip 70 is located at the other side area with respect to the array line L (i.e., the first side surface 70a and the third side surface 70c are located at opposite sides with respect to the array line L as the boundary between both the side areas).
However, in the above-described conventional LED module (light emitting element module), the chips are arranged on the board so that the side surfaces of the neighboring chips confront each other over the entire width of the side surface of the chips. That is, the chips are arranged so as to be completely aligned with one another in the longitudinal direction of the chips as shown in FIG. 7. Therefore, during the mount/demount process in which a chip is disposed at a predetermined position or when a defective chip is removed from the board, there is a risk that the chip concerned will come into careless contact with neighboring chips and disturb the arrangement of the chips or damage the neighboring chips. Further, during the process in which a new chip is die-bonded to a portion of the board from which a defective chip has been removed, there is also a risk that the new chip win come into careless contact with the neighboring chips and disturb the arrangement of the chips or damage the neighboring chips. Particularly when the light emitting elements are arrayed in higher density (i.e., the resolution of the LFD module is higher), it is more difficult to keep a sufficient chip interval between the neighboring chips, and thus it is more and more difficult to arrange a chip at a predetermined position and remove a defective chip. This win be described in more detail with reference to FIG. 7 which shows the conventional LED module.
In FIG. 7, r represents the chip edge margin, and s represents the inter-chip distance. The following table shows the possible maximum inter-chip distance s for the chip edge margins of 4 .mu.m and 8 .mu.m when the resolution of the LED module is set to 400 DPI, 600 DPI, 1200 DPI and 2400 DPI.
______________________________________ MAXIMUM INTER-CHIP DISTANCE s chip edge margin = 4 .mu.m chip edge margin = 8 .mu.m ______________________________________ 400 DPI 55.5 .mu.m 47.5 .mu.m 600 DPI 34.3 .mu.m 26.3 .mu.m 1200 DPI 13.0 .mu.m 5.0 .mu.m 2400 DPI 2.5 .mu.m unable ______________________________________
It is readily understood from the above table that as the resolution of the LED module is increased, the maximum possible inter-chip distanced is reduced. It is possible to mark each chip with an alignment mark and optically arrange each chip at a predetermined position while monitoring the mark. However, this technique also has a limitation in positioning of the chips.
Accordingly, as the light emitting elements of the LED module are arranged in higher density (as the resolution of the LED module increases) as described above, the probability increases that the arrangement of the neighboring chips will be disturbed or the neighboring chips will be damaged when a chip is disposed at a predetermined position or a defective chip is removed. Likewise, when a new chip is die-bonded to a portion of the board from which a defective chip has been removed, the probability that the arrangement of the neighboring chips will be disturbed or the neighboring chips will be damaged is also increased.